It all depends on how you're counting. For one, "open source" was not a phrase before 1998, so there is some retrofitting of Free Software projects. But also, there isn't a registry, it's rather difficult to be more than approximate with this. The article is very specific about their methodology, I'm only using one graph as a general example.
> "The database contains data from January 1990 until May 2007. Of this time
horizon, we analyze the time frame from January 1995 to December 2006. We omit
data before 1995 because it is too sparse to be useful"
> "Large distributions like Debian are counted as one project. Popular projects such as
GNU Emacs are counted as projects of their own, little known or obsolete packages
such as the Zoo archive utility are ignored"
So even though methodology is "very specific", it seems very incomplete/ inaccurate/ selective. Even Linux kernel, as per their source, started in 2005 (https://openhub.net/p/linux).
are (along with too many other projects) from way before Nov 1993, where "The Total Growth of Open Source" graph starts from 0.